Dimensional Box

This is a box. You can put an item inside or take an item out. The same box will appear later, so don't worry about coming back. Sincerely, a box lover.

— A sign introducing the box to the protagonist in the forest after the Ruins

Box sprite

A box can be used to store items, as there is a limit to how many items the protagonist can carry at a time. There are a few physical boxes before Alphys upgrades the protagonist's phone in the Neutral and True Pacifist Routes. After the protagonist's phone is upgraded, two dimensional boxes (one of which is the normal box) appears in the phone list and can be used to move items around. The dimensional boxes can be accessed at any point in the game. If the protagonist is on a Genocide Route, Alphys does not show up to upgrade the protagonist's phone; however, three additional boxes are placed to compensate for this.

Each box can store up to 10 items, so in the Pacifist and Neutral Routes, the protagonist can be in possession of a total of 28 items while in the Genocide Route, the protagonist can have a total of 18 items.

The first time the protagonist opens a box (the first of which is located in Snowdin), it contains a Tough Glove, which can be taken free of charge.

Trivia

If the protagonist tries to use the dimensional boxes in the Artifact Room the message "The box is aclog with the hair of a dog." is displayed, implying that the Annoying Dog is to blame. The protagonist is not able to use the boxes until they leave the room.

Items cannot be directly swapped between a box and the protagonist's inventory, only deposited and withdrawn. If all boxes and the player's inventory are full, and the player wants to exchange an inventory item for a box item, they must consume or drop an item first to make the switch possible.

The protagonist cannot use the dimensional boxes if they do not have any items in the inventory nor in the box.

A quote from the box lover is included in the trailer for the collector's edition release of Undertale for PlayStation 4 and PlayStation Vita.